This invention relates to an apparatus for the inspection of features on semiconductor devices to provide topographical data for purposes of specimen analysis.
The use of scanning electron microscopes (SEM) as a tool for the purpose of evaluating topographical features on semiconductors is well documented in the literature. Reference is made to Wells, et al, "Method for Examining Solid Specimens with Improved Resolution in the Scanning Electron Microscope (SEM)", Appl. Phys. Lett., vol. 23, No. 6, pp. 353-354 (September 1973); Wells, "Low-Loss Image for Surface Scanning Electron Microscope", Appl. Phys. Lett., vol. 19, No. 7, pp. 232-235 (October 1971); Utterback, "Semiconductor Dimensional Metrology using the Scanning Electron Microscope", Review of Progress in Quantitative Nondestructive Evaluation, vol. 7B, pp. 1141-1151, Plenum Press (1988).
In general, by the use of a SEM, an electron beam impinges on a specimen surface under examination. Details of the specimen surface are obtained by using detectors to detect either backscatter electrons or secondary electrons generated by the specimen surface. Normally, the E beam is focused to a fine spot by means of the microscope optic-electronics and it is then caused to scan the surface of the specimen. By various imaging techniques in conjunction with computer of the scanning pattern, a detailed examination of the topography of the specimen is obtained.
Also within the patent literature are a number of schemes using SEM techniques for purposes of testing specimens. Reference is made to U.S. Pat. No. 3,539,999 which uses a series of slotted plates, which are operated at a potential such that optimal secondary electron collection is insured. U.S. Pat. No. 3,714,424 employs a standard secondary electron detector with a collection grid electrically coupled to the stage. This system decreases electron current by an applied electric field to insure that only low energy secondary electrons will reach the detector. U.S. Pat. No. 4,011,450 also uses an applied electric field as an auxiliary electrode to increase secondary electron intensity and contrast for purposes of inspection of a sample. By using two or more collection electrodes, this scheme creates electric fields to pull electrons from the sample and push them toward the detector.
The collection of secondary electrons is also disclosed in U.S. Pat. No. 4,442,355. In that patent an in-lens secondary electron detector has a pipe electrode concentric with the beam axis. The electron microscope employs a grid to produce an accelerating field for secondary electrons. The scheme guides electrons without perturbing the beam. U.S. Pat. No. 3,736,424 also collects secondary electrons and the detection is enhanced by generating a filtering dielectric field at the sample. A specimen current detector is electrically coupled to the stage.
Difficulties in secondary electron detection are also dealt with in U.S. Pat. No. 4,743,757. Uneven detection is eliminated by the use of a split grid which is interposed between the specimen and the scintillator. An alternative grid technique is suggested in U.S. Pat. No. 4,596,929, which employs a pair of grids to improve detection efficiency by attracting electrons to the detector.
U.S. Pat. No. 3,646,344 also relates to an SEM specimen inspection scheme utilizing three grids to form a potential barrier of adjustable height for purposes of discriminating electrons emanating from regions of different potential at the specimen. As such, the scheme disclosed is an energy-filter detector. It defines a so called "window" which passes electrons, but employs a filter grid, which is biased to a potential above that of the beam. Thus, the presence of such a filter grid precludes electrons passing through to the detector element. The system employs a single detector with the scintillator held at ground. The system employing three grids physically extends below the sample surface and does not offer any adjustability in position. It will not work as a secondary electron detector and will not work at voltages below about 10 keV.
Importantly, the system is unsuitable for quantitative measurements. This is because the displacement of the scintillator from the filtered-sample access results in a nonuniform electron detection over the energy band width and collection angle. Thus, the system is not configured to function in an environment requiring three dimensional metrology.
U.S. Pat. No. 4,179,604 deals with the collection and detection of backscatter electrons and allows for the discrimination of detected electrons on the basis of their energy. This scheme permits the collection of low loss electrons and provides for improved energy discrimination. The scheme however is predicated on an erroneous assumption that electrons will be able to pass through a filter grid that is biased to a potential that actually exceeds that of the beam itself. This is impossible. Further, the patent indicates that the system could be constructed employing a filter grid with 400 mesh. Such a grid, however, would reduce the number of electrons that could pass by a factor of approximately 75%. Moreover, in this system the detector extends below the surface plane of the sample and is not designed for use in spatial discrimination of electron take-off angle. A single element detector is disclosed and consequently, the system cannot be used for stereophotometric imaging. The position of the detector is not adjustable and the detector element is always held at ground. Thus, the system cannot be used as a secondary electron detector. It also cannot be used for making three dimensional measurements of intact wafers at beam voltages below 10 keV.
While the art defines a variety of schemes advancements in techniques of sample inspection have not matched development of semiconductors of ever decreasing size. As semiconductor minimum features sizes shrink to below 0.5 .mu.m non-optical inspection techniques such as SEM must play an increasingly important role in the inspection and measurement of semiconductor wafers. With those very small feature sizes prior optical inspection techniques simply cannot provide sufficient resolution for reliable inspection. However, SEM techniques, as discussed herein, have a unique set of challenges and drawbacks. For example, due to the uncertainty in the origin of detected electrons in ordinary secondary electron imagining of surface structures, particularly in the vicinity of features edges, the ability to make accurate measurements is compromised to an unacceptable extent. Also, the use of an SEM poses physical constraints on the size of a sample that can be inspected without repositioning either the stage or the sample.
FIG. 1 illustrates positioning of conventional detectors relative to a specimen and the generation of both secondary electrons and backscattered electrons. FIG. 1 illustrates the conventional placement of elements for SEM detection. A pole piece of the SEM provides focusing for a primary electron beam (PE). The beam impinges on the surface of a specimen 10 having topographical features 12 such as metalization or trenches. The radius of the primary beam (PE) is illustrated in FIG. 1 by the radius (R). The impingement of the primary beam (PE) results in both a secondary electron emission from the specimen 10 and also the generation of backscattered electrons.
Secondary electron emission and the paths of those electrons is illustrated by lines SE-I, SE-II, and SE-III. Note also the generation of secondary electrons SE-IV from the SEM optics. Additionally, backscattered electrons are produced from the surface and illustrated as BSE-Is and BSE-II. The backscattered electron emission BSE-II produces, when those electrons strike the pole piece not only a reflected backscattered, BSE-III but also, a tertiary secondary electron SE-III. Reference is made to Peters, K. W. R. (1984) "SEM Contrast at High Magnification" in Microbeam Analysis--1984, ed, A. D. Romig, Jr. and D. B. Williams (San Francisco Press) pp. 135-139 which discusses electron discrimination and in particular Backscatter Is electrons. These electrons are also known as "Low-loss" or "no-loss" electrons since they do not have scattering events prior to detection. In that regard, Wells, O. C. (1971), "Low-loss Image for Surface Scanning Electron Microscope", API, 19, 232-23 and Reimer, L. (1985), "Scanning Electron Microscopy" Springer-Verlag (Springer-Verlag, Berlin Heidelberg), 139-142 discusses the characteristics of this specie of electrons. The problem of discrimination and collection of those electrons by a secondary electron detector, as illustrated in FIG. 1, is to discriminate between those secondary electrons generated by the specimen and those emanating from other factors. This is an unresolved problem in the art. While electron-material interaction modeling can help alleviate this difficulty, it is limited by the applicability of the energy loss equations the models must invariably employ. In the case of electron interactions with organic materials such as a photoresist, the equations are not generally sufficiently accurate. Moreover, effects such as substrate charging can affect penetration in ways which cannot be reliably predicted.
A more satisfactory approach to solving the problem is to develop electron detection techniques which eliminate the uncertainty in the origin of the detected electrons. For example, it would be highly desirable to selectively exclude electrons which penetrate through the sidewall of a feature such as a micro circuit line, i.e., exclude all electrons except those illustrated in FIG. 1 as BSE-Is, the low-loss or no-loss electrons. If this could be done then, it would be easier to determine where the true edge of that feature is located. The desirability of such techniques has been suggested by Utterback in "Semiconductor Dimensional Metrology Using the SEM", supra and by Rosenfield, "Analysis of Line Width Measurement Techniques Using the Low Voltage SEM", SPIE 775, 70, (1987). Filtering backscatter electrons for improved resolution was suggested very early in McMullen (1953), "An Improved Scanning Microscope for Opague Specimens", Proc. Inst. Elec. Eng. (London) Pt. B 100 245-259.
Despite such suggestions of principles and theory, there remain in the art unresolved technical deficiencies which have precluded the practical application of such concepts in the setting of semiconductor inspection during fabrication as an on-line, operative scheme.